Automatic volume control circuit



Jan. 24, 1939. ANDREATTA 2,144,601

AUTOMATIC VOLUME CONTROL CIRCUIT I Filed NOV. 30, 1935 75 1NVEZ\TOR. ALFONS ANDREATTA ATTORNEY.

Patented Jan. 24, 1939 v NETED STATES PATENT OFFIQE AUTOIVIATIC VOLUDIECONTROL CIRCUIT Alfons Andreatta, Berlin, Germany, assignor toAllgemeine Elektricitatz Gesellschaft, Berlin, Germany ApplicationNovember 30, 19-35, Serial No. 52,323

In Germany February 20, 1935 1 Claim. (Cl. 250-20) Automatic volumecontrol in radio apparatus stance, O to 6 volts. As already pointed outabove is obtained, in g neral, in that the grid bias of only one half ofthe control potential is used for amplifier, or detector, tubes isvaried in relation the control of grid 8. If the bias of tube at to thehigh frequency potential applied to the zero control potential is equalto 1 volt, there '5 input of the receiver. The tubes in this case thusappears at grid 8 a variation of l to 4 5 are so constructed thatvarious grid biasing povolts, and at grid 6 a variation of -1 to '7volts. tentials correspond to different degrees of am- In order toobtain a Wide range of control it is plification. In the drawing, theusual circuit is desirable to utilize fully these voltages for therepresented in Fig. 1. Figs. 2 and 3 show the control. However, thiscannot be done in the 10 two different modifications of the invention.circuit shown, since the Voltage drop at resistor 10 The controlpotential at resistor 2 and pro- 9 is in opposition to the controlpotential. It is duced by tube l is applied, across resistors 3 andnecessary to provide a. bias of, for instance, 3 to the first controlgrid 6 and across resistor volts at the biasing resistor ll] of tube Iin view I to the second control grid 3 of tube 5. In the of theconsiderable tone frequency potential at circuit shown, grid 8 is notsupplied by the entire the grid of the tube. 15 control potential, butreceives only a part thereof At the control potential 6 volts, grid 6has a for instance one half of the control potential. potential of 6(resistor 2) +3 (resistor Ill) 1 A resistor Q through which a voltagedrop is (res to 9) VO- 3 has the potential produced by the cathodecurrent, supplies a fur- 3 (from resistor 2) +3 (resistor l9) 1(resistor ther grid bias for the tube 5. The tube 5 has 9) 1 volt. Inplace of a control of 1 to 7, 20 three biasing potentials applied: or 1to 3 volts there occurs a control in the (l) A negative bias derivedfrom the resistor range from -4 to 1 or 0 volt, i. e. the control 9 dwhose lu depends upon the c th de fails at the second grid. It,therefore, follows Current that despite the considerable number ofelements (2) The control potential at resistor 2 which is nd de pite theco trol of two d an ent y 25 like ise negative. unsatisfactory controlwill be obtained with the (3) A positive bias at resistor H! whichtechknown circuit, and which is far from sufficient nically cannot besuppressed and which is in for compen g most of the fadi p ewmenaopposition to the potentials I and 2. The aforementioned drawbackis overcome by Ordinarily, the resistors are so dimensioned theautomatic volume control circuit described 30 that at a controlpotential equal to zero, both in the following. With this circuit it ispossible grids of t tube m b cgntrglled h a l negato attain the widestpossible control determined tive bias of, for instance, one voltimpressed by the exponential characteristic of the tube or thereon inorder to prevent grid current. This is tubes s and y the ControlPotential produced accomplished by increasing the Voltage drop at atresistor 2. In the new circuit the direct cur- 35 resistor s by, forinstance, one volt above that n w through r i or 9 is maintained conatresistor it. If a high-frequency potential is stant so that also aconstant voltage drop through applied to the input resistor 4 Which isamplified t e es t W be produced. The direct current in the tube 5 anddetected in the diode path ll, Potential pp aring at resistor I0 isconstant as the grid potentials of tube 5 become negative inaforementioned. The negative grid bias results 40 View of the controlpotential appearing at the from the difference of the direct currentpotenresistor 2. tials produced at resistor 9 and at resistor H],

Consequently. 3118 P te current of tube 5 Will be in addition to thecontrol potential at resistor 2. decreased AS a result thereof thereoccurs a Since in view of the arrangement of the circuit,

further Q -i f the girld Voltage of tube the potentials at the resistors9 and Ill are connamfly P f that as a result of the stant, the variationof the negative grid biasing flecrrcase p f Current the voltage qpotential depends only upon the potential at rethrough the resistor 9becomes smaller. As will t 2 th t a t 1 b be seen from the followingdata, this voltage or so a m Xlmum con m w} e variation is undesirable,since it acts against the a In the example of the calcplatlon aboYe 50control potential and decreases the range of acglven there Wonk} 1n factbe obtained for gnd 6 a control potential of l to '7 volts (as comticnof the control.

It will be assumed that the available variation pared Wlth to volts 0fthe hltherto known of the high-frequency potential at resistor 4proarran nt), a r grid 8 a control p ntial 55 duces at resistor 2 acontrol potential of, for inof l to 4 volts-(in the hitherto known ar-55 rangement no control at all was obtained through rid 8).

The circuit diagrams 2 and 3 show examples for the practical applicationof the invention. In the circuit of Fig. 2, the emission current of theend tube is at the same time passed as additional direct current throughthe resistor 9 whereby practically a constant voltage drop throughresistor 9 is obtained since the emission current of the end tube islarger by one order than the emission current of tube 5.

The plate current of end tubes as used at this time in radio apparatuswith volume control is approximately 40 mA, and the plate current of acontrolled tube is 3 to 4 mA. While in the above example the voltage atresistor 9 fluctuates on account of the cathode current Within thelimits between 0 and 3 volts, the voltage produced by the cathodecurrent remains practically constant when applying the invention. Thoseskilled in the art will realize that resistor 9 should have its re--sistance reduced to a value such that the total current flowtherethrough will provide the desired normal operating bias for the grid6.

A further example of applying the invention is shown in Fig. 3. In thiscircuit a practically constant voltage drop is produced through theresistor 9 by the additional current of the voltage divider of thescreen grid. This is accomplished in that the screen grid potentiometerconsisting of the resistors l2 and i 3 is not grounded at the pole as isusually done, but is placed at the cathode of the tube 5 so that thepotentiometer lies in series to the resistor 9. The extent to which thebasic voltages existing at resistor 9 shall be independent of thecathode current of tube 5 simply depends upon a correspondingdimensioning of the resistors l2 and I 3.

The automatic volume control according to the invention, may also beobtained in another way, for instance, the exciting current of the fieldcoil of a dynamic loudspeaker may be passed through the resistor 9. Thishas for its effect an especially great independence from the cathodecurrent of tube 5 since the latter is only & of the exciting current.

What is claimed is:

In a radio receiver comprising at least one sig nal transmission tubeprovided with at least a signal input electrode, a cathode and an outputelectrode, an impedance in the space current path of the tube developinga first direct current voltage, a signal rectifier including animpedance in its space current path for developing a second directcurrent voltage which varies in magnitude with the signal amplitude, asource of direct cur rent voltage of relatively fixed magnitude, saidsignal input electrode being connected to said cathode through anautomatic gain control path including at least a portion of saidrectifier impedance, said fixed source and at least a portion of saidfirst impedance arranged in series relation, the voltage of said fixedsource being in polarity opposition to the first and second directcurrent voltages and the latter two voltages biasing said signal inputelectrode negatively, and means for maintaining said first voltagesubstantially independent of the changes in negative bias of said signalinput electrode, said means comprising a direct current source connectedto said first impedance in such a manner that the current thereof flowsthrough the impedance, the magnitude of the last source beingsufficiently greater than that of the transmission tube space currentthat the said first direct current voltage remains substantiallyconstant in value.

ALFONS ANDREATTA.

